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Syn-Magmatic Shear Zones, Alagoinhas Pluton, Borborema Province Brazil

 

Roberto Weinberg, Monash University, Australia

Gorki Mariano, Federal University of Pernambuco, Brazil

 

 

Copyright 2004-2011 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise, subject to inclusion of this copyright notice and acknowledgment of the source URL: users.monash.edu.au/~weinberg.

 

I would very much appreciate an email stating how this material will be used: Roberto Weinberg, Monash University, Australia. Thanks, RW.

 

DISCLAIMER. The material on this website has not undergone the scrutiny of Monash University and does not conform to its corporate web design. It is entirely based on a free-spritied, curiosity-driven research effort by the author, and therefore in no way expresses the official position of the University.

 

Pedra Furada, Venturosa
Pedra Furada, close to the town of Venturosa, Alagoinhas Pluton (24L 0739267E 9051982N)
In the page entitled Syn-Magmatic Deformation in Alagoinhas Pluton several features related to syn-magmatic deformation of a single outcrop were described, including boundinage, shearing and melt extraction. This page expands on that with more features from the same pluton as well as others.

 

a) Shear Zone and Enclaves (24L 0743377E 9062224N, Alagoinhas Pluton pt 20) 
Shear zone through enclave
Figure 1. Shear zone through an enclave. Note the felsic material in between the two enclave parts, and the shearing of feldspar phenocrysts in the granite.

 

syn-magmatic shear zone
Figure 2. Mafic enclave with a broken and rotated tip. Crack filled in by felsic fractionated granite. The tip is rotated into a shear zone (see Fig. 2). 		syn-magmatic shear zone
Figure 3. Dextral shear zone (parallel to pen)  breaking off the tip of enclave. Shear zone is marked by the rotation of feldspar phenocrysts from their main orientation parallel to main enclave elongation.
syn-magmatic shear zone
Figure 4. Same shear zone as in Figs 1 and 2, showing the dextral disruption of another enclave.

 

b) Subtle Sinistral Shear Zones Defined by Phenocryst Rotation

 

(Alagoinhas Pluton close to Pedra Furada pt 19)
syn-magmatic shear zone
Figure 5. Shear zone (east-west in photo) defined by the rotation of phenocrysts giving rise to a sinistral drag fold. 		syn-magmatic shear zone
Figure 6. Shear zone (east-west in photo) defined by the rotation of phenocrysts  from the south,defining a sinistral drag fold, but truncating phenocrysts in the north side.
syn-magmatic shear zone
Figure 7. Shear zone (est-west in photo) defined by the rotation of phenocrysts giving rise to a sinistral drag fold.

 

syn-magmatic shear zone, phenocryst
Figure 8. Shear zone (east-west in photo) defined by the rotation of phenocrysts  from the south into a sinistral drag fold, but truncating the trend defined by phenocrysts in the north side.
 yn-magmatic shear zone, phenocryst
Figure 9. Shear zone (diagonal from upper left) defined by the rotation of phenocrysts. Note that outside the main shear zone, on the south side of photo, phenocrysts are orineted in two main directions and locally tiled such as to suggest shearing.